Universal serial buses (USB), being one of the most popular transmission means today that can be disposed on various surfaces, can be adapted for almost all kinds of devices that are to be used for different operations, such as data transmission or charging, etc. The amount of power that can be provided by a USB connector may not be enough for an electronic device. Therefore, there are cables available to allow using two USB connectors to supply power. FIG. 3 is a prior art illustrating a USB cable has two inputs and one output (Dual-input USB Cable). The two inputs are for connecting to USB ports of a computer. The output is for connecting to an external hard drive. Once the USB cable is connected, the external drive may be able receive adequate power supply from the computer through the Dual-input USB Cable.
However, carrying a dual-input USB Cable is not convenient. Further, if power supply provided through one USB port is adequate, the other USB port may be used as a backup power supply for redundancy purpose. It is also desirable to have the power supplied from two independent sources, instead of from the same computer or USB power supply. This is particularly important for device that does not have internal battery. Further, as there are many non-standard compliant USB power supplies, protection needs to be built-in to avoid damage to electronic circuits.